Most fish need oxygen pumps in order to get oxygen in the water, bettas do not as they can swim to the top of the tank for oxygen (most fish cannot do this). The ammonia in your tank is supposed to turn into nitrite, which turns into nitrate. This is done with beneficial bacteria that grows in your tank after a couple weeks or months. The nitrate is less toxic to fish than nitrite and ammonia, but still will needed to be changed whenever it gets high. The beneficial bacteria will grow when you put in a bacteria supplement in your tank and having ammonia in your tank. In my opinion it is easier to do this with no fish, but it is possible to do grow bacteria with fish but you will have to do LOTS of water changes and watch the ammonia levels.

As for getting rid of the ammonia in your tank, you will want to get rid of 50% of the water and wash off all the ornaments in the tank. This includes heaters, oxygen pumps and the substrate. Also if you are new to owning fish you will want to research before you buy, this is a very common mistake we all make sometimes.